1. Field of the Invention
The present invention relates to a method of adjusting drive current of a magnetism sensor driven by constant current and an electronic azimuth meter having a magnetism sensor, particularly to restraining a dispersion in a sensitivity of an electronic azimuth meter having a magnetism sensor and promoting the sensitivity.
2. Description of the Prior Art
As shown, by a block circuit diagram of FIG. 5, a conventional electronic azimuth meter is constituted by a magnetism sensor 10, a sensor drive circuit 20 for driving the magnetism sensor 10 by constant current, a selecting circuit 30 for selecting the magnetism sensor driven by the sensor drive circuit 20, CPU 51 for determining an azimuth by subjecting a magnetic field output signal of the magnetism sensor 10 to a digital signal processing, ROM 52 stored with a firmware program and the like for controlling operation of the electronic azimuth meter, RAM 53 stored with processing data and the like, a key input circuit 54 for inputting information of start/stop of the electronic azimuth meter and the like, a display apparatus 55 for displaying information of an azimuth and the like, a display drive circuit 56 for driving the display apparatus 55, an oscillating circuit 57 and a dividing circuit 58.
The magnetism sensor 10 is constituted by two pieces of magnetism sensor bridges arranged orthogonally to each other, that is, an X-axis magnetism sensor and a Y-axis magnetism sensor. The respective magnetism sensor bridge is a magnetism sensor module constituted by four pieces of magnetoresistive elements connected in Wheatstone bridge type.
The sensor drive circuit 20 is a circuit for supplying constant current of drive current to the magnetism sensor 10. That is, the sensor drive circuit 20 supplies constant current for driving, for example, constant current of a constant value having a magnitude of 1 [mA] to the four pieces of magnetoresistive elements connected in Wheatstone bridge type.
Characteristics of the magnetoresistive elements constituting the magnetism sensor 10 are made to be as uniform as possible since the magnetism sensor 10 is driven by the constant value of the constant current. However, owing to restriction in view of fabrication, actually, the characteristics of the magnetoresistive elements constituting the magnetism sensor 10 cannot be made to be uniform sufficiently.
For example, bridge resistance Rb of the magnetoresistive element constituting the magnetism sensor 10 and sensitivity Sd of the magnetoresistive element are dispersed independently from each other. Specifically, as shown by FIG. 6, whereas the bridge resistance Rb is dispersed as 1.0 [kxcexa9], 1.5 [kxcexa9], 2.0 [kxcexa9], in contrast to the respective bridge resistance Rb, the sensitivity Sd is dispersed as 4.0 through 8.0 [(mV/V)/(kA/m)]. When current Ib having a magnitude of 1 [mA] is supplied to these samples, a dispersion of bridge voltage Vb (=Ibxc3x97Rb) becomes 1.0 through 2.0 [V] and accordingly, sensitivity Sa (=Sdxc3x97Vb) of the magnetism detecting circuit is dispersed as 4.0 through 16.0 [mV/(kA/m)] or four times as large as the dispersion.
When the dispersion of the sensitivity Sa of the magnetism detecting circuit is large in this way, according to a portable electronic azimuth meter having a magnetism sensor, the following problem is posed since the power source is a battery. First, owing to the battery drive, an upper limit of the bridge voltage Vb generated at the bridge resistance Rb is restricted by the battery voltage. Specifically, when a lithium battery is used, the battery is operated from an initial stage of 3.0 [V] to a final stage of 2.0 through 2.4 [V] and accordingly, the bridge voltage Vb is restricted to be equal to or lower than the value. Further, a maximum value of sensor drive current Ib necessarily becomes Vb/Rb provided by dividing the bridge voltage Vb by the bridge resistance Rb. Since the bridge resistance Rb of the magnetism sensor is dispersed, in the case of conventional constant current drive, the drive current Ib is set from a maximum value of the bridge resistance Rb. Under the condition of the drive current Ib set in this way, in the case of a sample in which the bridge resistance Rb is minimized and the sensitivity Sa is minimized by the dispersion in the magnetism sensor, the sensitivity Sa of the magnetism detecting circuit is reduced. When an electronic azimuth meter is constituted by such a magnetism sensor, there poses a problem that accuracy of the electronic azimuth meter is deteriorated.
It is a problem to be resolved to reduce a dispersion in a sensitivity Sa of a magnetism detecting circuit caused in the case of driving a magnetism sensor comprising magnetoresistive elements in a bridge constitution by constant current, to a dispersion in a sensitivity Sd of the magnetism sensor per se.
In order to resolve the above-described problem, according to a magnetism detecting circuit for driving, by current, a magnetism sensor comprising magnetoresistive elements in a bridge constitution, drive current Ib of the magnetism sensor is made adjustable in accordance with a bridge resistance value Rb of the magnetism sensor.
That is, a method of adjusting drive current of a magnetism sensor according to the invention is constituted by the following steps.
(1) a step of respectively measuring bridge resistance of the X-axis magnetism sensor and bridge resistance of the Y-axis magnetism sensor;
(2) a step of respectively calculating the drive current set value of the X-axis magnetism sensor and the drive current set value of the Y-axis magnetism sensor from measured values of the bridge resistance of the X-axis magnetism sensor and the bridge resistance of the Y-axis magnetism sensor: and
(3) a step of respectively storing the drive current set value of the X-axis magnetism sensor and the drive current set value of the Y-axis magnetism sensor to the drive current storing means.
Further, a number of the drive current set value is N and is represented by an integer provided by equally dividing a certain range by N, further specifically, N is set to 16.
Further, an electronic azimuth meter according to the invention is constituted by magnetism sensor means comprising an X-axis magnetism sensor and a Y-axis magnetism sensor, drive current set value storing means for storing drive current set values, sensor driving means having a drive current source capable of supplying drive current adjustable in a range of N steps for storing drive current in correspondence with the drive current set values stored to the drive current set value storing means to the magnetism sensor means, A/D conversion means for converting an analog output of the magnetism sensor means into a digital output, azimuth calculating means for outputting an azimuth signal by subjecting the digital output of the A/D conversion means to predetermined calculation, displaying means for displaying an azimuth based on the azimuth signal of the azimuth calculating means, and sensor controlling means for controlling the means.
Further, a number of the drive current set value is N and is represented by an integer provided by equally dividing a certain range by N, further specifically, N is set to 16.
Further, an electronic azimuth meter according to the invention is constituted by magnetism sensor means comprising an X-axis magnetism sensor and a Y-axis magnetism sensor, drive current set value storing means for storing drive current set values, sensor driving means having a drive current source capable of supplying drive current adjustable in a range of N steps for storing drive current in correspondence with the drive current set values stored to the drive current set value storing means to the magnetism sensor means, A/D conversion means for converting an analog output of the magnetism sensor means into a digital output, azimuth calculating means for outputting an azimuth signal by subjecting the digital output of the A/D conversion means to predetermined calculation, displaying means for displaying an azimuth based on the azimuth signal of the azimuth calculating means, and sensor controlling means for controlling the means, further, there is used the drive current set value storing means stored with the drive current set values provided by the following steps.
(1) a step of respectively measuring bridge resistance of the X-axis magnetism sensor and bridge resistance of the Y-axis magnetism sensor; and
(2) a step of respectively calculating the drive current set value of the X-axis magnetism sensor and the drive current set value of the Y-axis magnetism sensor from measured values of the bridge resistance of the X-axis magnetism sensor and the bridge resistance of the Y-axis magnetism sensor.
Further, a number of the drive current set value is N and is represented by an integer provided by equally dividing a certain range by N, further specifically, N is set to 16.
Further, an electronic azimuth meter comprising: a magnetism sensor comprising an X-axis magnetism sensor and a Y-axis magnetism sensor; a drive current set value storing circuit to store drive current set values; a sensor driving circuit having a drive current source capable of supplying drive current adjustable in a range of N steps for storing drive current in correspondence with the drive current set values stored to the drive current set value storing circuit to the magnetism sensor; an A/D converter to converte an analog output of the magnetism sensor into a digital output; an azimuth calculator to output an azimuth signal by subjecting the digital output of the A/D converter to predetermined calculation; a display to display an azimuth based on the azimuth signal of the azimuth calculator; and a sensor controller to control the devices.
Further, an electronic azimuth meter comprising: a magnetism sensor comprising an X-axis magnetism sensor and Y-axis magnetism sensor; a drive current set value storing circuit to store drive current set values; a sensor driving circuit having a drive current source capable of supplying drive current adjustable in a range of N steps for storing drive current in correspondence with the drive current set values stored to the drive current set value storing circuit to the magnetism sensor; an A/D converter to convert an analog output of the magnetism sensor into a digital output; an azimuth calculator to output an azimuth signal by subjecting the digital output of the A/D converter to predetermined calculation; a display to display an azimuth based on the azimuth signal of the azimuth calculator; and a sensor controller to control the device; wherein the drive current set values are stored to the drive current set value storing cirtcuit by the following steps of:
(1) a step of respectively measuring bridge resistance of the X-axis magnetism sensor and bridge resistance of the Y-axis magnetism sensor; and
(2) a step of respectively calculating the drive current set value of the X-axis magnetism sensor and the drive current set value of the Y-axis magnetism sensor from measured values of the bridge resistance of the X-axis magnetism sensor and the bridge resistance of the Y-axis magnetism sensor.
Further, the electronic azimuth meter characterized in that a number of the drive current set values is N and is represented by an integer provided by equally dividing a certain range by N.
Further, the electronic azimuth meter characterized in that N is 12.